1. Field of the Invention
This invention relates to electrical heaters comprising PTC elements.
2. Summary of the Prior Art
PTC compositions, i.e. compositions which exhibit positive temperature coefficients of resistance, are well known, and heaters and other electrical devices which comprise at least one PTC element (i.e. an element composed of a PTC composition) are also well known. Reference may be made for example to U.S. Pat. Nos. 2,978,665; 3,243,753; 3,351,882; 3,412,358; 3,413,442; 3,591,526; 3,673,121; 3,793,716; 3,823,217; 3,858,144; 3,861,029; 3,914,363 and 4,017,715; and U.S. Patent Office Defensive Publication No. T 905001. Reference may also be made to commonly assigned U.S. patent application Ser. Nos. 601,550 (now U.S. Pat. No. 4,188,276), 601,638 (now U.S. Pat. No. 4,177,376), 601,639 (now abandoned), 638,440 (and the CIP thereof Ser. No. 775,882), 732,792 (now abandoned), 750,149 (now abandoned), 873,876 now U.S. Pat. No. 4,246,468 and 965,345 now abandoned. The disclosure of each of these publications and applications is hereby incorporated by reference.
The PTC heaters which are most widely used in practice are strip heaters which comprise an elongate strip of a conductive polymer PTC composition, the strip having in contact therewith (generally embedded therein) two or more parallel electrodes whose ends can be connected to a source of electrical power, the strip and the electrodes being surrounded by an electrically insulating jacket. The strip heater is, for example, wound around a metal pipe which is to be maintained at a controlled elevated temperature, and the pipe and heater may be surrounded by a layer of thermal insulation. Reference may be made for example to the Thermal Design Guide published by the Chemelex Division of Raychem Corporation (H 50190 505 B5 1/78). Although such heaters have proved extremely useful, the active power output which can be obtained from such a heater is not as high as is desirable. Attempts to improve the active power output by varying the resistivity of the PTC composition (and therefore the resistance of the heater in a given geometry) give only a small increase in useful power output and involve other disadvantages, for example increased current inrush and/or shorter heater life. Strip heaters which have been used heretofore at normal supply voltages (generally 120 or 240 volts) have passive power ratings of 7 to 50 watts per foot and active power ratings of 4 to 10 watts per foot, with the ratio of passive power to active power being from 2:1 to 5:1. The terms "passive power rating" is used herein to denote a theoretical power output given by the term V.sup.2 R.sub.0, where V is the nominal intended supply voltage and R.sub.0 is the resistance of the heater at 70.degree. F. The term "active power output" is the measured power output of the heater with one major surface thereof in contact with metal substrate which is at some temperature related to the intended use of the heater, for heaters for freeze protection for pipes, and with the heater powered at its normal operating voltage.